This invention relates to strengthened bond coats for thermal barrier coatings that protect metal substrates, and in particular to provide improved spallation resistance for such thermal barrier coatings. This invention further relates to articles, in particular turbine engine components, having a metal substrate that use such improved bond coats with such thermal barrier coatings.
The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature alloys have been achieved through the formulation of iron, nickel and cobalt-base superalloys, though components formed from such alloys often cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to provide turbine engine components with an environmental coating that inhibits oxidation and hot corrosion, or a thermal barrier coating (TBC) system that thermally insulates the component surface from its operating environment. TBC systems typically include a ceramic layer adhered to the component with a metallic bond coat that also inhibits oxidation and hot corrosion of the component surface.
Coating materials that have found wide use as TBC bond coats and environmental coatings include overlay alloy coatings such as MCrAlX where M is iron, cobalt and/or nickel and X is hafnium, zirconium, yttrium, tantalum, platinum, palladium, rhenium, silicon or a combination thereof. Also widely used are aluminide diffusion coatings which are formed by a diffusion process, such as pack cementation, above pack, vapor phase, chemical vapor deposition (CVD) or slurry coating processes. The diffusion process results in the coating having two distinct zones or layers, the outermost of which is an additive layer containing an environmentally-resistant intermetallic represented by MAl, where M is nickel, cobalt, and/or iron, depending on the substrate material. Beneath this additive layer is a diffusion zone or layer comprising various intermetallic phases that form during the coating process as a result of diffusional gradients and changes in elemental solubility in the local region of the substrate.
Following deposition, the surface of a bond coat is typically prepared for deposition of the ceramic layer by cleaning and abrasive grit blasting to remove surface contaminants, roughen the bond coat surface, and chemically activate the bond coat surface to promote the adhesion of the ceramic layer. Thereafter, a protective oxide scale is formed on the bond coat at an elevated temperature to further promote adhesion of the ceramic layer. The oxide scale, often referred to as a thermally grown oxide (TGO), primarily develops from selective oxidation of the aluminum and/or MAl constituent of the bond coat, and inhibits further oxidation of the bond coat and underlying substrate. The oxide scale also serves to chemically bond the ceramic layer to the bond coat.
The bond coat used to adhere the thermal barrier coating to the metal substrate can be extremely important to the service life of the thermal barrier coating system that protects the metal substrate. During exposure to the oxidizing conditions within a gas turbine engine, bond coats inherently continue to oxidize over time at elevated temperatures, which gradually depletes aluminum from the bond coat and increases the thickness of the oxide scale. As a result of the thermal expansion mismatch between the bond coat and the oxide scale, as well as the scale growth process and relative mechanical properties at temperature, thermal cycling leads to stresses that cause ratcheting or rumpling of the scale into the bond coat. Eventually, the scale reaches a critical thickness and a high level of rumpling that leads to spallation of the ceramic layer by delamination either at the interface between the bond coat and the oxide scale, or at the interface between the oxide scale and the thermal barrier coating. Once spallation has occurred, the component can deteriorate rapidly, and therefore must be refurbished or scrapped at considerable cost.
Because of the cost associated with refurbishing or scrapping such components, there is a continuous need to improve the spallation resistance of such thermal barrier coatings through improvements in the bond coat. Beneficial results have been achieved by incorporating oxides into the bond coat, as taught by commonly assigned U.S. Pat. No. 5,780,110 (Schaeffer et al), issued Jul. 14, 1998; U.S. Pat. No. 6,168,874 (Gupta et al), issued Jan. 2, 2001; and U.S. Pat. No. 6,485,845 (Wustman et al), issued Nov. 26, 2002. In the Schaeffer et al patent, a submicron dispersion of oxide particles is placed on the surface of the bond coat to inoculate the bond coat oxide. The inoculated bond coat can be preoxidized to form a mature alpha-alumina scale, or a thermal barrier coating can be immediately deposited, during which the inoculated bond coat forms the desired mature alpha-alumina scale. However, inoculating the bond coat surface prevents or at least limits the type of surface preparation that the bond coat can undergo prior to deposition of the thermal barrier coating. For example, bond coat surface cleaning and roughening by grit blasting and electropolishing are precluded by the presence of the oxide particles at the bond coat surface.
In the Gupta et al patent, this complication of the Shaeffer et al method is avoided by codepositing the diffusion bond coat and oxide particles. However, codepositing according to the Gupta et al method cannot readily control the types and morphology of oxides incorporated into the bond coat.
In the Wustman et al patent, the oxide particles are preferentially entrapped in the bond coat by depositing the oxide particles on the surface of the component prior to forming the bond coat. The deposition of the bond coat causes the oxide particles to thus become dispersed in the outer surface region thereof. Wustman et al indicates that suitable oxide particle sizes for dispersion can be less than about 45 microns, although smaller or larger particles could also be used. The improved spallation resistance of the Wustman et al system is attributed to: (1) limiting the diffusion of elements from the metal substrate to the bond coat/thermal barrier coating interface, thus limiting the potential for these elements to form oxides that are detrimental to adhesion of the ceramic layer; (2) creating a tortuous path for crack propagation along the bond coat/thermal barrier coating interface, and therefore acting to limit crack propagation along this interface; (3) providing preferred sites for improving the anchoring of the ceramic layer, and/or that local modification of the bond coat surface and/or chemistry to provide for an improved bond between the ceramic layer and the bond coat; or (4) a combination of these explanations.
In the Wustman et al system, the large particles present can potentially allow relatively high surface areas to be exposed to the oxidizing atmosphere, thus causing rapid internal oxidation, and subsequently poor oxidation resistance. Control of the particle distribution can be difficult or potentially impossible using the Wustman et al system. There is also the potential inability to create a distribution of extremely fine (i.e., nanometer to micron size) particles in the Wustman et al system.
Bond coat strengthening to limit rumpling and subsequent spallation is usually achieved by addition of oxidatively reactive elements. See commonly-assigned U.S. Pat. No. 5,975,852 (Nargaraj et al), issued Nov. 2, 1999, (NiAl overlay bond coat to which is optionally added one or more reactive elements such as yttrium, cerium, zirconium or hafnium) and U.S. Pat. No. 6,291,084 (Darolia et al), issued Sep. 18, 2001 (predominantly beta-phase NiAl overlay bond coating with limited additions of zirconium and chromium). However, oxidatively reactive elements are difficult to incorporate and control in diffusion coatings. The level of oxidatively reactive elements required for strengthening can also be potentially high enough to degrade the oxidation resistance of the bond coat. Dispersion strengthening of the bond coat, be it an overlay coating such as MCrAlY and especially a diffusion coating with components that do not actively participate in the oxidation process could potentially increase the overall performance of the bond coat.
Accordingly, it is still desirable to be able to further improve the spallation resistance of the thermal barrier coating through modifications of the bond coat. In particular, it would be desirable to modify the bond coat to enable strengthening thereof to limit bond coat ratcheting or rumpling and subsequent thermal barrier coating spallation, as well as to improve overall oxidation resistance through these strengthening improvements. It would be further desirable to be able to strengthen the bond coat by using components that do not actively participate in the oxidation process, especially where the bond coat is a diffusion coating.